1. Technical Field
The present invention relates to a constant-temperature piezoelectric oscillator provided with a frequency voltage control circuit for keeping the setting temperature of a thermostatic oven and compensating the temperature difference between the setting temperature and the peak temperature of a piezoelectric vibrator, and a method of manufacturing the constant-temperature piezoelectric oscillator.
2. Related Art
As crystal oscillators, which are frequency control device used for mobile communication devices or transmission communication devices, there have been known oven-controlled piezoelectric oscillator for outputting waves with highly stabilized frequency without being affected by external temperature variations. In recent years, the market requires small-sized, light-weight, and low power-consumption oven-controlled piezoelectric oscillators for various types of devices.
JP-A-10-303645 (Document 1) discloses an oven-controlled piezoelectric oscillator with reduced power consumption, and FIG. 12 is a block diagram thereof. The oven-controlled piezoelectric oscillator is provided with a heating element 62 for heating a voltage controlled piezoelectric oscillator 63 inside a thermostatic oven 61, an in-oven temperature-sensitive element 64 disposed inside the thermostatic oven 61, and an out-oven temperature-sensitive element 65 for detecting the ambient temperature outside the thermostatic oven. The oven-controlled piezoelectric oscillator is further provided with a control voltage generation circuit 67 for controlling the voltage of the voltage controlled piezoelectric oscillator 63 based on the temperature information from the out-oven temperature-sensitive element 65 to thereby vary the frequency thereof, and a temperature control circuit 66 for controlling the temperature of the heating element 62 based on temperature information of the temperature difference between the out-oven temperature-sensitive element 65 and the in-oven temperature-sensitive element 64.
In the oven-controlled piezoelectric oscillator shown in FIG. 12, the temperature inside the thermostatic oven has some variation, and therefore, the voltage-controlled piezoelectric oscillator 63 is used as the piezoelectric oscillator housed inside the thermostatic oven 61 so that the frequency can be controlled by supplying the voltage generated by the control voltage generation circuit 67 based on the temperature information from the temperature-sensitive elements.
Further, JP-A-2007-251366 (Document 2) discloses a constant-temperature crystal oscillator detecting the variation in ambient temperature and performing temperature control. FIG. 13 is a block diagram of a temperature compensation circuit of a circuit board disposed inside the oscillator having a single oven structure. The temperature compensation circuit is composed of a temperature sensor 71, an amplifier 72, an adder 73, a target temperature setting input terminal 74, an amplifier 75, an integrator 76, an adder 77, an amplitude adjusting resistor 78, a heater power supply terminal 79, a heater 80, a transistor 81, an adder 82, a frequency correction input terminal 83, an amplifier 84, a resistor 85, a variable-capacitance diode 86, a crystal vibrator 87, and a frequency output terminal 88. It is desirable that the temperature sensor 71 and the heater 80 are disposed on the circuit board and arranged near to the crystal vibrator 87.
The difference between the voltage output of the amplifier 72 based on the detection by the temperature sensor 71 and a target temperature input voltage 74 is detected by the adder 73 to output a first difference signal. The variation in the first difference signal due to the temperature variation is extracted, and the output of the adder 77 having the extracted signal and the first difference signal as the inputs lowers the collector voltage of the transistor 81 to thereby increase the current to the heater 80 so that the temperature rises if the ambient temperature drops. In contrast, if the ambient temperature rises, the current to the heater 80 is decreased. In order for correcting the frequency of the crystal vibrator, a correcting voltage value (a frequency correction signal) is set previously, and is input to the adder 82. According to the disclosure, a second difference signal is formed by detecting the difference from the extracted signal, and the vibration frequency of the crystal vibrator is controlled based on the second difference signal.
However, since the oven-controlled piezoelectric oscillator disclosed in Document 1 is a piezoelectric oscillator having temperature-sensitive elements disposed respectively inside and outside the thermostatic oven, and controlling the control voltage generation circuit and the temperature control circuit based on at least either one of the external temperature information and the internal temperature information, there arises a problem that the frequency stability of the oven-controlled piezoelectric oscillator is insufficient although the power consumption of the heating element for heating the inside of the thermostatic oven can be reduced.
Further, the crystal oscillator disclosed in Document 2 is made for attempting to enhance the frequency stability of the constant-temperature crystal oscillator by detecting the variation in the ambient temperature for performing the temperature control. However, it is premised on the fact that oven temperature is adjusted to the peak temperature of the crystal vibrator similarly to the existing constant-temperature crystal oscillator, and there arises a problem that the adjustment requires a large amount of work.
Besides the problems described above, in order for obtaining the highly stabilized piezoelectric oscillator, a requirement of tuning of the piezoelectric vibrating element with a high accuracy in the peak temperature becomes increasingly severe, and therefore, degradation in productivity and increase in production cost are incurred.